Neurodevelopment disorders are often accompanied by significant language impairments;however, most research on children with these disorders has assessed only the outcomes of language learning (e.g., vocabulary level achieved at a particular age), leaving unexplored the processes by which language is learned. Focusing exclusively on outcomes might mask differences among particular syndromes, whereas understanding the processes underlying the resulting learning will allow for targeted and potentially more effective interventions. Fragile X syndrome is the leading inherited cause of intellectual disability. It is caused by a mutation in the FMR1 gene and the resulting lack of its protein, FMRP, which affects synaptic connectivity. Language development is markedly delayed in children with fragile X syndrome, with a syndrome- specific profile of deficits as well as considerable within-syndrome variability. Syntactic development appears to lag behind cognitive development in this population;however, little is known about the specificity of delays in syntactic comprehension, the sources of that difficulty, and the impact those delays may have on other aspects of language development. The proposed dissertation will address these gaps. It is hypothesized that deficits in syntactic comprehension are in part due to impaired auditory working memory and that the former will disrupt the ability to use syntactic forms to interpret novel lexical verbs in the ways in which typically developing children do, thereby further hindering language learning. Paradigms designed for young typically developing children will be used to study the constructs of interest and make comparisons among boys with fragile X syndrome, boys with idiopathic autism spectrum disorder, and boys with typical development. Specific Aim 1 will determine the nature of the deficits in syntactic comprehension experienced by boys with FXS and the syndrome-specificity of those deficits. This aim will be addressed with a looking-while-listening comprehension task (Study 1). Specific Aim 2 will examine how the syntactic knowledge tested in Study 1 can be extended to novel verbs. This will be assessed in the context of a syntactic bootstrapping preferential-looking task using infrared eye-tracking (Study 2). Specific Aim 3 will determine how auditory working memory and expression of FMRP relate to syntactic comprehension and syntactic bootstrapping performance in Study 1 and Study 2 among boys with fragile X syndrome. Characterizing syntactic comprehension and its efficacy as a platform for further language learning in boys with FXS will be informative for understanding the range of variability in the fragile X syndrome phenotype. This research will contribute to our understanding of the connection between the linguistic phenotype of fragile X syndrome and its genetic underpinnings by addressing the role of cognitive processes in language development while setting the stage for targeted interventions. PUBLIC HEALTH RELEVANCE: This study will characterize an important aspect of language development in boys with fragile X syndrome: the role of auditory working memory and grammatical development in supporting the acquisition of new words. By characterizing specific aspects of syntactic comprehension in boys with fragile X syndrome and the ways in which boys with fragile X syndrome extend syntactic patterns to novel verbs, this project will set the foundation for targeted interventions and illuminate genetic and cognitive underpinnings of language development.